1. Field of the Invention
The present invention is directed to the field of internal combustion engine analyzers. More particularly, the present invention is directed to that portion of the above-chemistry of the exhaust gases generated by an engine or a furnace as a measure of the air/fuel ratio of the combustion mixture being provided to the engine or furnace. More particularly still, the present invention is directed to that portion of the above-noted field which is concerned with the measurement of the partial pressure of oxygen within the exhaust gases generated by an internal combustion engine as a measure of the air-to-fuel ratio of the combustion mixture being provided to the engine. With greater particularity still, the present invention is directed to that portion of the above-noted field which is connected with the provision of a sensor mixture by changing an electrical characteristic in response to changes in the partial pressure of oxygen present in such exhaust gases. More particularly still, the present invention is directed to that portion of the above-noted field which is concerned with the provision of a partial pressure of oxygen responsive sensor which may be used within an internal combustion engine exhaust gas environment and which has a range of operation from air/fuel ratios having a fuel content richer than stoichiometry to air/fuel ratios having a fuel cnotent leaner than stoichiometry.
2. Description of the Prior Art
The above-noted copending commonly assigned patent application describes an exhaust gas sensor for use in an internal combustion engine exhaust gas environment. According to the application, the sensor utilizes cobalt monoxide ceramic material, maintained at a temperature in excess of about 900.degree. C, to respond to the partial pressure of oxygen within the exhaust gas produced by a combustion process, maintained, for example in a furnace or an internal combustion engine, operated with an air-to-fuel ratio which is fuel-lean compared to a stoichiometric mixture (hereinafter referred to as a "lean mixture"). The sensor is described as being used as an input device for an air/fuel ratio controller for the associated engine. Such a sensor, while capable of giving satisfactory results, nevertheless is subject to a pair of problems which cause such a sensor to be somewhat impractical as an automotive internal combustion exhaust gas sensor.
The first of these problems is caused by the tendency of cobalt monoxide ceramic material to decompose to form cobalt and oxygen at elevated temperature in the presence of very low quantities of oxygen. This condition is normally found in the internal combustion engine exhaust gases generated by a fuel-rich combustion mixture (hereinafter referred to as a "rich mixture"). For example, decomposition will begin to occur at oxygen partial pressures associated with an air/fuel ratio of about 14 to 1 with decomposition increasing in severity as the air to fuel ratio decreases. Operation of an internal combustion engine in an automotive environment normally entails brief excursions of the cmobustion mixture from a lean mixture to a rich mixture. Such excursions may intentionally occur when the vehicle is started after the engine has cooled to the ambient temperature or upon aaceleration of the vehicle. Such excursions also may occur intentionally under certain other operating conditions and may accidentally occur when the engine is misadjusted or requires readjustment. The decomposition of a cobalt monoxide ceramic material used as an exhaust gas sensor would cnostitute a catastropic failure of the system and must be avoided.
The successful performance of the cobalt monoxide ceramic material sensor requires that the temperature of the cobalt monoxide ceramic material be maintained at a relatively high temperature to avoid a phase change conversion of the cobalt monoxide (CoO) material to Co.sub.3 O.sub.4 a material which does not exhibit the desired electrical characteristic dependence on partial pressure of oxygen. This material phase change, which is the second of the above-noted problems, occurs at intermediate levels of elevated temperature in the presence of quantities of oxygen which normally would be associated with engine operation with a lean mixture. As the level of the partial pressure of oxygen increases, the temperature at which the phase change will begin to occur increases. Thus, as the quantity of oxygen would be predicted to increase, as would be the case of lean mixtures having decreasing fuel content, the temperature of the sensor would have to be elevated to prevent the undesired phase change. However, as the phase change is reversible, small operational excursions into lower temperature or elevated partial pressures of oxygen can be tolerated.
Thus, while the sensor described in the above-noted application performs well under certain conditions, partial pressure of oxygen range is too narrow to permit the use of the sensor without additional devices to protect the sensor when engine operation results in a sensor environment which exceeds the range. Furthermore, the minimum operating temperature of the cobalt monoxide ceramic material is very high and lower minimum temperature would be advantageous.
It is therefore an object of the present invention to provide an improved sensor material. It is a further object of the present invention to provide an improved cobalt monoxide based material in which the cobalt monoxide will begin to undergo the phase change to another form of cobalt oxide, which will hereinafter be referred to as the spinel form, at a lower temperature for a given partial pressure of oxygen. It is also an object of the present invention to provide an additive or alloy material for the cobalt monoxide ceramic material which additive or alloy material will not significantly alter the electrical response of the cobalt monoxide ceramic material to changes in partial pressure of oxygen. It is a further and specific object of the present invention to provide an additive or alloy material for inclusion in a cobalt monoxide ceramic material which will increase the ability of the ceramic material to resist decomposition, at elevated temperature, in the exhaust gases generated by combustion of rich mixtures. It is a still further object of the present invention to provide such an additive or alloy material which not substantially alter or diminish the ability of the cobalt monoxide ceramic material to respond electrically to changes in the partial pressure of oxygen of its environment. It is a further and particular object of the present invention to provide a class of material which may be alloyed with cobalt material prior to the fabrication of a ceramic sensor material which behaves electrically as cobalt monoxide ceramic material but which is able to withstand decomposition at elevated temperature in gaseous environments having very low concentrations of free oxygen comparable to the exhaust gases produced by combustion of rich mixtures and which will be resistant to conversion to a spinel form at relatively lower elevated temperatures in the presence of substantially greater quantities of free oxygen as may be present in the combustion gases produced by combustion of lean mixtures. It is also an object of the present invention to provide a process for fabricating such exhaust gas sensor ceramic materials.